Plastic lenses have advantages not possessed by inorganic glass lenses, such as lightweight, safety, processability, dyeability, and the like.
Polydiethylene glycol bis(allyl carbonate) ("CR-39", a trade name of PPG Co.) has been widespread as a material for plastic lenses because of its excellence in various characteristics but has a low refractive index n.sub.D as 1.50 and does not therefore suffice for needs of further reduction in weight and dimensions of plastic lenses.
There are several reports on polymers having increased refractive indices (n.sub.D =ca. 1.60) which are obtained by using a residue of bisphenol A, etc. in a monomer skeleton as disclosed in JP-A-No. 61-72748 and JP-A-No. 62-195357 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, these polymers involve disadvantages fatal to use as optical lenses. That is, the casting monomer undergoes shrinkage from mold dimension on polymerization (curing), failing to obtain a desired surface shape as designed, and the cured product is liable to suffer from optical strain due to residual stress or orientation.
In order to overcome these problems on polymerization casting, it has been proposed to previously increase the density of the casting liquid before cast molding or to lengthen the distance between crosslinking points to thereby alleviate contraction. However, these techniques of using a prepolymer are not easy to actually carry out because the casting liquid is highly apt to gel on prepolymerization and difficult to control so as to have a desired viscosity. Moreover, the resulting prepolymer is unsatisfactory in storage stability.